Color image processing apparatus and color image forming apparatus

ABSTRACT

A color image processing apparatus having at least patch image output units which output patch image data, based on patch data in a storage area, a patch data extractor which extracts read patch data by reading a patch image formed according to the patch image data, using a color scanner, a patch data processor which estimates the record gradation of patch data, based on the read patch data, and a gradation corrector which corrects a given color image signal, based on the estimated record gradation and patch data. The apparatus provides gradation correction based on gradation estimated from patch data, thus making it possible to reproduce gradation and color well.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 11-242119, filed Aug. 27,1999, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a color image processing apparatus, andmore particularly, to a color image processing apparatus which adjustsparameters for image processing, using a patch image.

Image forming apparatuses, such as copying machines and printers, haverecently been adapted to mainly do color printing. Accordingly, printcolor is required to have precisely specified gradation characteristics.For example, a copying machine performs color conversion to convert RGBimage data input from a scanner to CMY image data to be output on aprinter using color material. To duplicate an image so that the originaltone of color is faithfully reproduced, color conversion characteristics(color conversion parameters and a color conversion table) must beadjusted properly.

To duplicate a color image faithfully, gradation characteristics mustalso be adjusted properly. When passing through a sensor or a filter,image data input from a scanner suffers distortion for R, G, and B. Whena printer is provided with the same image data, images which differ ingradation and color are output from time to time due to differentenvironments (different temperatures and humidities), time-dependentchanges, and differences between individual products. Especially becausethe gradation characteristics of a printer sensitively change inresponse to various factors, a mechanism which properly adjusts theoutput gradation characteristics and color conversion characteristics ofthe printer must be installed in the printer to keep reproducing animage well.

A method for determining color conversion parameters is known whichcollects multicolor patch RGB-CMY data pairs by making an apparatus toread a color patch sample output therefrom and finds by the least-squaremethod the coefficient of each term of a polynomial for converting RGBdata to CMY data. The method, which considers image processorcharacteristics other than color conversion to be like a black box, isbased on the idea that ideally, an image can be reproduced faithfullyonly by color conversion, because black box inverse corrections are madeusing color conversion.

A method is also known which color conversion parameter setting arefixed under an assumption and adjusts output gradation characteristicsin the same way as described above. That is, the method collectsmultigradation patch C′, −C, M′, −M, Y′, and −Y data pairs by making anapparatus to read a color patch sample output therefrom and finds by theleast-square method the coefficient of each term of gradation correctionequations for C, M, and Y.

However, it is difficult to output an image with satisfactorilyreproduced gradation and color even using the above-described adjustingmethods. This is because image processing performed by equipment from ascanner to a printer comprises the steps of color conversion,blackening, gradation processing (quasi intermediate gradationprocessing), screen processing, etc. in which image data suffersnonlinear distortion with different characteristics.

Thus a problem with conventional image forming apparatuses it that it isdifficult to generate all parameters at a time which are used forcorrecting nonlinear distortion due to color conversion, blackening,gradation processing, screen processing, etc.

Another problem with conventional equipment is that it is difficult tocorrectly estimate record gradation for characteristics of individualequipment and incorporate the estimated record gradation into parametersfor processing of individual signals.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a color imageprocessing apparatus and a color image forming apparatus which output acolor image with well-reproduced gradation and color reproducibility byestimating record gradation or the like using patch image data andincorporating it in parameters.

According to the present invention, a color image processing apparatuscomprising: patch image output means for outputting patch image datagenerated from patch data stored in storage areas; patch data extractingmeans for receiving read data obtained by reading, by means of ascanner, a patch image represented by the patch image data output by thepatch image output means; gradation estimating means for estimating therecord gradation of the patch data, from the read patch data extractedby the patch data extracting means; and correction output means forcorrecting an input color image signal on the basis of the recordgradation estimated by the gradation estimating means and the patch datastored in the storage area, and for outputting the corrected signal.

According to the present invention, using such an arrangement, recordgradation is estimated from patch data extracted from read patch data,that is, patch data which is output and then captured, using a scanneror the like. By doing so, tables for correcting image processing, takinginto account a gradation characteristic tendency unique to equipment arecreated, thus providing a color image processing apparatus which makesit possible to make fine gradation corrections for individual equipment.

According to the present invention, a color image processing apparatuscomprising: first patch image output means for outputting first patchimage data on the basis of the first patch data for creating a pulsewidth selection table stored in the first storage area; first patch dataextracting means for reading, using a scanner, a first patch imagerepresented by first patch image data output by the first patch imageoutput means, and for extracting first patch data in accordance with thefirst patch image; first gradation estimating means for estimating therecord gradation of the first read patch data, from on the first readpatch data extracted by the first patch data extracting means; pulsewidth selection table setting means for setting a pulse width selectiontable on the basis of the record gradation of the first read patch dataestimated by the first gradation estimating means and the first patchdata; second patch image output means for outputting second patch imagedata, using the pulse width selection table set by the pulse widthselection table setting means and on the basis of the second patch datafor creating a gradation correction table stored in the second storagearea; second patch data extracting means for reading, using a scanner, asecond patch image represented by second patch image data output by thesecond patch image output means, and for extracting second patch data inaccordance with the second patch image; second gradation estimatingmeans for estimating the record gradation of the second read patch dataon the basis of the second read patch data extracted by the second patchdata extracting means; gradation correction table setting means forsetting a gradation correction table on the basis of the recordgradation of the second read patch data estimated by the secondgradation estimating means and the second patch data; third patch imageoutput means for outputting third patch image data, using the pulsewidth selection table set by the pulse width selection table settingmeans and the gradation correction table set by the gradation correctiontable setting means and on the basis of the third patch data forcreating a color conversion table stored in the third storage area;second patch data extracting means for reading, using a scanner, a thirdpatch image represented by third patch image data output by the thirdpatch image output means, and for extracting third patch data accordingto the third patch image; color conversion table setting means forsetting a color conversion table on the basis of the third read patchdata extracted by the third patch data extracting means and the thirdpatch data; and correction output means for correcting a given and inputcolor image signal on the basis of the pulse width selection table setby the pulse width selection setting means, the gradation correctiontable set by the gradation correction table setting means, and the colorconversion table set by the color conversion table setting means, andfor outputting the corrected signal.

According to the present invention, based on gradation estimated in sucha way, image processing tables, that is, pulse width selection,gradation correction, and color conversion tables are created one afteranother. By doing so, the adjustable parameter tables can bedispersively provided at a plurality of key points in an imageprocessing flowchart to create each table according to a relativelysimple target or rule, so that a color image processing apparatus can beprovided which feeds a color image with well-reproduced gradation andcolor.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a block diagram illustrating a configuration of an imageprocessor according to the present invention.

FIG. 2 is a block diagram illustrating a configuration of a patch dataprocessor according to the present invention.

FIG. 3 a block diagram illustrating another configuration of the patchdata processor according to the present invention.

FIG. 4 is a block diagram illustrating an example of an image formingapparatus using an image processing apparatus according to the presentinvention.

FIG. 5 is flowchart generally illustrating execution of the presentinvention.

FIG. 6 is a flowchart for creating a pulse width selection table.

FIG. 7 is a flowchart for creating a gradation correction table.

FIG. 8 is a flowchart for creating a color conversion table.

FIG. 9 is a plan view showing patch images for creating a pulseselection table and a gradation correction table.

FIG. 10 illustrates data sampling from one patch.

FIG. 11 is a graph showing an example of a relationship between the arearate calculated by polynomial approximation and pulse width.

FIG. 12 is a graph showing an example of a relationship between the arearate calculated by polynomial approximation and 8-bit gradation level.

FIG. 13 is a plan view showing an example of patch image for creating acolor conversion table.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, embodiments of the present invention aredescribed below.

Configuration and Operation of Image Processor

Below are described a configuration of an image processor 36 whichcorrects characteristics, using a patch image characterizing the presentinvention and characteristic correction by the processor.

FIG. 1 is a block diagram illustrating the configuration of the imageprocessor according to the present invention. As shown in FIG. 1, theimage processor according to the present invention, which is connectedwith a color scanner 1 and color printer 2 when used, consists of thefollowing components connected together: an image processing controller51, a preprocessor 52, a patch data extractor 53, a color conversiontable setting unit 54, patch data memory C 56, a patch data creator C57, a color converter 58, a blackening unit 59, a gradation correctiontable setting unit 60, a gradation corrector (output γ corrector) 61,patch data memory B 62, a patch image creator B 63, a quasi intermediategradation processor 64, a pulse width selection table setting unit 65,patch data memory A 66, a pulse width selector 67, a patch image creatorA 68, a screen processor 69, and a patch data processor 70, or the hubof the present invention.

The function of each component is described according to a ordinary flowof data as observed when an image is duplicated by a digital copyingmachine using the image processor. At first, the RGB image data on anoriginal read by the color scanner 1 is input to the preprocessor 52.The preprocessor 52 makes shading and input γ corrections to the imagedata fed from the scanner. The color converter 58 converts the inputimage data, expressed by RGB color space, to image data expressed by CMYcolor space. The blackening unit 59 calculates the amount of black-colormaterial (K) from the CMY data (blackening material creation) andcorrects the amount of CMY according to the value of K (black-colorreplacement). After blackening, the output γ corrector 61 makes agradation correction to the CMY image data for each color channel. Thequasi intermediate gradation processor 64 performs quasi intermediategradation processing, such as multivalue error expansion. For example, a16-value recording system performs 16-value error expansion. In thiscase, each image data pixel output only takes 16 values from 0 to 15,expressed using 4 bits. The pulse width selector 67 selects the pulsewidth of a recording laser drive signal for each of levels 0 to 15. Thescreen processor 69 converts the image data, converted to a pulse widthsignal, to a recording pattern which is given a screen angle for eachcolor channel. The color printer (recording engine) 2 outputs the imagesubjected to screen processing onto paper. Other blocks in FIG. 1 aredescribed below together with table creation.

Structure of Digital Copying Machine Using Image Processor

The structure and operation of a digital copying machine is describedbelow which uses the above-described image processor of the presentinvention. FIG. 4 is a block diagram schematically illustratingelectrical connections and a control signal flow in a digital copyingmachine according to the present invention. In FIG. 4, a control systemconsists of three CPUS: a main CPU (central processing unit) 91 in amain controller 30, a scanner CPU 100 in the color scanner 1, and aprinter CPU 110 in the color printer 2.

The main CPU 91 has bi-directional communicates through common RAM(random access memory) with the printer CPU 110. The main CPU 91 issuesan operational direction to the printer CPU, and the printer CPU 110returns a status to the main CPU. The printer CPU 110 and scanner CPU100 have serial communications with each other. The printer CPU issuesan operational direction to the scanner CPU, and the scanner CPU 100returns a status to the printer CPU.

An operation panel 40, which has a liquid crystal display 42, operationkeys 43, and a panel CPU 41 connected with the display and keys, isconnected with the main CPU 91.

The main controller 30 consists of the main CPU 91, ROM (read-onlymemory) 32, RAM 33, NVRAM 34, the common RAM 35, the image processor 36,a page memory controller 37, page memory 38, a printer controller 39,and printer font ROM 121.

The main CPU 91 controls the entire system. A control program and thelike are stored in the ROM 32. The RAM 33 temporarily stores data.

The NVRAM 34, nonvolatile random access memory backed up by a battery(not shown), maintains stored data if the power is turned off.

The common RAM 35 is used for bi-directional communications between themain CPU 91 and printer CPU 110.

The page memory controller 37 stores image information in the pagememory 38 and reads it therefrom. The page memory 38 with an areasufficient to store a plurality of pages of image information is adaptedso that data into which image data from the color scanner 1 iscompressed can be stored for each page.

Font data corresponding to print data is stored in the printer font ROM121. The printer controller 39 expands print data from externalequipment, such as a personal computer, into image data at a resolutionequivalent to resolution data included in the print data, using fontdata stored in the printer font ROM 121.

The color scanner 1 comprises the scanner CPU 100, which controls theentire system; ROM 101 in which a control program and the like arestored; RAM 102 for storing data; a CCD driver which drives a colorimage sensor 15; a scanning motor driver 104 which controls rotation ofa scanning motor moving a first carriage; and an image corrector 105.

The image corrector 105 comprises an analog digital converting circuitwhich converts R, G, and B analog signals output from the color imagesensor 15 to digital signals, a shading correcting circuit whichcorrects variations in the threshold level relative to an output signalfrom the color image sensor 15 due to color image sensor 15 variations,an ambient temperature change, or the like, and line memory whichtemporarily stores a digital signal subjected to shading correctionwhich is fed from the shading correction circuit.

The color printer 2 comprises the printer CPU 110 which controls theentire system, ROM 111 in which a control program and the like arestored, RAM 112 for storing data, a laser driver 113 which drives asemiconductor laser oscillator 60, a polygon motor driver 114 whichdrives a polygon motor 54 of an exposure apparatus 50, a transfercontroller 115 which controls transfer of paper P by a transfermechanism 20, a process controller 116 which controls a process ofcharging, development, and transcription performed using a chargingapparatus, a developing roller, and a transfer apparatus, a fixationcontroller which controls a fixing apparatus 80, and an optioncontroller 118 which controls an option.

The image processor 36 which has a function characterizing the presentinvention, page memory 38, printer controller 39, image corrector 105,and laser driver 113 are connected together using an image data bus 120.

Processing Performed by Image Processor Using Patch Image

Referring now to the drawings, processing which is performed by an imageprocessor of the present invention, using a patch image is describedbelow.

FIG. 5 is a flowchart illustrating a series of steps in which imageprocessing parameter tables of the present invention are created.

In an embodiment of the present invention, the above-described imageprocessor creates the pulse width, gradation correction, and colorconversion tables in the order shown in FIG. 5. That is, first,preprocessing input γ characteristics are made linear (S11), and thenthe pulse width selection table is created using, for example, a patchtable (S12). Next, the gradation correction table is created based onthe pulse width selection table obtained, using a patch image, forexample (S13). Finally, the color correction table is created based onthe gradation correction table obtained, using, for example, a patchimage (S14). In the embodiment, the tables are sequentially created, onewhich has the lowest degree of freedom and can be created according tothe simplest policy first, and moreover, the following table is based onthe preceding table. This allows an input image to be reproducedcorrectly although a simple creating method does not.

Referring to a flowchart, steps in which the three tables are createdusing patch images are described in detail below.

FIG. 6 shows a pulse selection table creating flowchart. First, a patchimage for creating a pulse width selection table is output (S21). Patchdata for creating a pulse width selection table is stored in patch datamemory A 66. The patch data, representing pulse widths, is set so that aplurality of patches are laid out which are provided by changing thepulse widths at regular intervals for C, M, Y, and K. The patch imagecreator A 68 creates patch image data for creating a pulse widthselection table, using the patch data. FIG. 9 shows an example of apatch image configuration. The patch image data is fed to the screenprocessor 69 (S22). In response to a control signal from the imageprocessing controller 51, the screen processor 69 selects image datafrom the patch image creator A 68 as input. After subjected to screenangle processing, the patch image data is output onto paper (S23).

The output patch image for creating a pulse width selection table isread by the scanner 1 (S24). In this case, using the image processingcontroller, the input γ characteristics of the preprocessor 52 arechanged so that they are linear, not as usual (S25). The reason why thisstep is taken is as follows. It is desirable that patch data read bedirectly proportional to the quantity of light reflected from a patchimage to estimate record gradation (for example, the are rate) from thepatch data read using a patch data processor 70, described later.However, it is sometimes desirable that the input γ characteristics bedirectly proportional to a power of the quantity of light reflected.After subjected to preprocessing, patch image data input from thescanner is fed to the patch data extractor 53. Based on presetinformation on patch print layout, the patch data extractor 53 samplespixel data to extract the R, G, and B values of each patch (S26). Tocancel the effect of noise, many pixels in rectangular formation are cutout of the middle of a patch, and the total of them is calculated. Eachof the R, G, and B values is divided by the number of pixels cut to takean average (see FIG. 10).

From the extracted patch data, the record gradation of the patch imageis estimated (S27).

Record gradation can be estimated by various methods. For example, atleast three methods are conceivable. The first method finds the C, M, Y,and K area rates, the second finds both record area rate and recorddensity and selects either as required, and the third sets a pluralityof levels of record gradation and estimates gradation, referring to areference table which is created for each record gradation level. Inaddition, various modified methods are available.

First, a method which finds the record area rate is described below. TheC, M, Y, and K record area rates of a patch image are estimated usingthe following: $\begin{matrix}\left. \begin{matrix}{C = {255 - R}} \\{M = {255 - G}} \\{Y = {255 - B}} \\{K = {255 - {\left( {R + G + B} \right)/3}}}\end{matrix} \right\} & (1)\end{matrix}$where R, G, B, C, M, Y, and K are each expressed using eight bits.

Finally, the pulse width selection table setting unit creates a pulsewidth selection table, based on the estimated area rate and patch datawhich is stored in the patch data memory A. The relationship between theestimated area rate and pulse width is approximately found as thepolynomial of n-th degree named Expression 2 (S28). The least-squaremethod is used to calculate coefficients a_(k) in Expression 2.$\begin{matrix}{P = {\sum\limits_{k = 0}^{n}\quad{a_{k}x^{k}}}} & (2)\end{matrix}$

(x: area rate, p: pulse width)

FIG. 11 shows an example of a curve representing the relationshipbetween the estimated area rate and pulse width. Pulse selection tablevalues are calculated by dividing the area rate axis into as many equalsegments as recorded values (for 16-value error expansion, the axis isdivided into 16 equal segments) and finding the pulse widthcorresponding to each division point using Expression 2 (S29).

Although the number of gradation levels is limited to, for example, 16,a pulse width selection table is created according to a simple,easy-to-keep rule that a laser drive pulse width is assigned so that therecord area rate linearly changes for each level (that is, each of thelimited number of gradation levels represents an area rate), asdescribed above. This makes it possible to keep good gradationresolution over the entire density range, thus reproducing gradationwithout density breaks or density skips.

Below is described the way to find both record area rate and recorddensity and select either as required. FIG. 2 shows an example of thepatch image processor 70, which consists of a density estimator 71, anarea rate estimator 72, and a selector 73 which receives output from thedensity estimator and area rate estimator and selects the record arearate or record density as specified by a gradation estimating methodspecifier 74. For this configuration, processing performed by thedensity estimator 71 is represented by the following:C=−log10(r/255)M=−log10(G/255)Y=−log10(B/255)K=−log10[(R+G+B)/3/255]  (3)where R, G, B, C, M, Y, and K are each expressed using eight bits.

The selector 73 selects the estimated density obtained using theabove-described equation and estimated area rate as specified by thegradation estimating method specifier 74.

Then the pulse width selection table setting unit 65 sets a pulse widthselection table in the same way as described above.

Using FIG. 3, below is described the way to set a plurality of levels ofrecord gradation and estimate gradation, referring to a reference tablewhich is created for each record gradation level. As shown in FIG. 3,first and second gradation estimators 81 and 82 each have a firstgradation estimating reference table 85, first table setting means 86which make settings in the first gradation estimating reference table, asecond gradation estimating reference table 83, and second table settingmeans 84 which make settings in the second gradation estimatingreference table. The selector receives output from the first and secondgradation estimators and outputs either as specified by the gradationestimating method specifier 88.

The first and second gradation estimators 81 and 82 may be theabove-described density estimator 71, area rate estimator 72, or otherrecord gradation estimating means. The gradation estimating referencetables 85 and 83 can be set at will by the gradation table setting units86 and 84. Such a configuration makes it possible to provide an imageprocessor materializing a digital copying machine with higher gradationreproducibility by switching between a plurality of gradation estimatorsaccording to use condition.

Using the gradation correction table creating flowchart in FIG. 7, aprocedure for setting a gradation correction table is described below.First, a patch image for creating a gradation correction table isoutput. Patch data for creating a gradation correction table is storedin the patch data memory B 62. The patch data, representing 8-bitgradation levels to be fed to the quasi intermediate gradation processor64, is set so that a plurality of patches are laid out which areprovided by changing the gradation levels at regular intervals for C, M,Y, and K. The patch image creator B 63 creates patch image data forcreating a gradation correction table, using the patch data (S31). Thelayout of patch images is almost the same as in FIG. 9. The patch imagedata is fed to the quasi intermediate gradation processor 64. Inresponse to a control signal from the image processing controller 51,the quasi intermediate gradation processor 64 selects image data fromthe patch image creator B 63 as input. After subjected to quasiintermediate gradation processing (S32), pulse width selection using theabove-described set pulse width selection table (S33), and screen angleprocessing (S34), the patch image data is output onto paper (S35).

The output patch image for creating a gradation correction table is readby the scanner (S36). In this case, using the image processingcontroller 51, the input γ characteristics of the preprocessor 52 arechanged so that they are linear, not as usual. The reason is the casewith the above-described pulse width selection table creation. Patchimage data input from the scanner is fed through the preprocessor 52 tothe patch data extractor 53 (S37). Based on preset information on patchprint layout, the patch data extractor 53 samples pixel data to extractthe R, G, and B values of each patch (S38). To cancel the effect ofnoise, many pixels in rectangular formation are cut out of the middle ofa patch, and the total of them is calculated. Each of the R, G, and Bvalues is divided by the number of pixels cut to take an average (FIG.10).

From the extracted patch data, the record gradation of the patch imageis estimated (S39). As described above, at least three methods forestimating record gradation, including those of FIGS. 2 and 3, areavailable: the first method which finds C, M, Y, and K area rates, thesecond which finds both record area rate and record density and selectseither as required (see FIG. 2), and the third which sets a plurality oflevels of record gradation and estimates gradation, referring to areference table created for each record gradation level. In addition, asis the case with pulse width selection table setting, various methodsare possible, including steps extracted from those methods (for example,a step of estimating record density only) and combinations of the steps.

Finally, the gradation correction table setting unit creates a gradationcorrection table, based on the estimated record gradation (for example,the area rate) and patch data which is stored in the patch data memory B(S41). The relationship between the estimated area rate and 8-bit outputgradation level is approximately found as the polynomial of n-th degreenamed Expression 4. The least-square method is used to calculatecoefficients b_(k) in Expression 2 (S40). $\begin{matrix}{L = {\sum\limits_{k = 0}^{n}\quad{b_{k}x^{k}}}} & (4)\end{matrix}$

(x: area rate, L: 8-bit output gradation level)

FIG. 12 shows an example of a curve representing the relationshipbetween the 8-bit output gradation level and estimated area rate.Gradation correction table values are calculated by dividing the arearate axis into 256 equal segments and finding the 8-bit gradation levelcorresponding to each division point using Expression 4.

Finally, using the flowchart for creating a color conversion table ofFIG. 8, a procedure for creating a color conversion table is describedbelow. First, a patch image for creating color conversion table isoutput (S51). Patch data for creating a color conversion table is storedin the patch data memory C. The patch data, representing a 8-bit arearate each for C, M, and Y, is set so that a plurality of patches arelaid out which are provided by combining C, M, and Y when the area rateis changed at regular intervals. The patch image creator C creates patchimage data for creating a color conversion table, using the patch data.FIG. 13 shows an example of arrangement of patches for creating a colorconversion table. The patch image data is fed to the blackening unit. Inresponse to a control signal from the image processing controller, theblackening unit selects image data from the patch image creator C asinput. After subjected to blackening (S52), gradation correction usingthe above-described set gradation correction table (S53), quasiintermediate gradation processing (S54), pulse width selection using theabove-described set pulse width selection table (S55), and screenprocessing (S56), the patch image data is output onto paper (S57).

Then the output patch image for creating a color conversion table isread by the scanner (S58). In this case, the input γ characteristics ofthe preprocessor are set for ordinary copying (S59). This is bothbecause for color conversion, RGB data itself fed from the preprocessoris input and because starting the procedure shown by the table creationflowchart with the patch data representing the area rate eliminates theneed for area rate estimation. After subjected to preprocessing, patchimage data input from the scanner is fed to the patch data extractor 53(S60). Based on preset information on patch print layout, the patch dataextractor 55 samples pixel data to extract the R, G, and B values ofeach patch. To cancel the effect of noise, many pixels in rectangularformation are cut out of the middle of a patch, and the total of them iscalculated. Each of the R, G, and B values is divided by the number ofpixels cut to take an average (FIG. 10).

Finally, the color conversion table setting unit 54 creates a colorconversion table, based on the extracted patch data and patch data whichis stored in the patch data memory C. The RGB-CMY color conversionrelationship is approximately found as the matrix equation namedExpression 5 (S62). The least-square method is used to calculate colorconversion matrix elements Dij in Expression 5. $\begin{matrix}{\begin{bmatrix}C \\M \\Y\end{bmatrix} = {\begin{bmatrix}D_{00} & D_{01} & D_{02} & D_{03} & D_{04} & D_{05} & D_{06} & D_{07} & D_{08} & D_{09} \\D_{10} & D_{11} & D_{12} & D_{13} & D_{14} & D_{15} & D_{16} & D_{17} & D_{18} & D_{19} \\D_{20} & D_{21} & D_{22} & D_{23} & D_{24} & D_{25} & D_{26} & D_{27} & D_{28} & D_{29}\end{bmatrix}\quad\begin{bmatrix}R \\G \\B \\R^{2} \\G^{2} \\B^{2} \\{RG} \\{GB} \\{BR} \\1\end{bmatrix}}} & (5)\end{matrix}$

Color conversion values are calculated by dividing the RGB color spaceinto equal portions at right angles to each axis and finding C, M, and Yvalues corresponding to the RGB coordinates of each division grid point.

The pulse width selection table, gradation correction table, and colorconversion table are created in that order. That is, these tables aresequentially created, one which has the lowest degree of freedom and canbe created according to the simplest policy first, and then tables whichhave higher degree of freedom are created, taking the already createdtable into account. Although the number of gradation levels is limitedto, for example, 16, a pulse width selection table is created accordingto a simple, easy-to-keep rule that a laser drive pulse width isassigned so that the record area rate linearly changes for each level.Thus gradation resolution can be kept good over the entire densityrange.

Based on patch data stored in the storage area using such anarrangement, patch image data is output, and patch data is extractedfrom reading patch data read by a scanner or the like from a readingpatch image formed according to the patch image data. An imageprocessing apparatus which makes gradation correction best suited forthe equipment possible is provided by estimating record gradation fromthe patch data. Moreover, a digital copying machine using the imageprocessing apparatus provides high gradation and color reproducibility.

Because image processing controlling means are provided which switchinput γ characteristics set for ordinary copying to linear input γcharacteristics during preprocessing preceding patch data extraction,the present invention makes it possible to estimate the area ratenecessary for table creation with high accuracy when a patch image isread using a scanner to create pulse width selection table and agradation correction table.

CONCLUSION

As described above, the present invention makes it possible to estimategradation more accurately by selecting gradation information, such asthe area rate, gradation density, or a combination of these, from patchdata as the case may be and referring to created tables. Thus gradationcharacteristic variations from equipment to equipment are correctedaccurately, providing a color image processing apparatus which allowsgradation and color to be reproduced well.

Based on a gradation estimated in such a manner, image processingparameter tables, that is, a pulse width selection table, a gradationcorrection table, and a color conversion table are created in thatorder. By doing so, the adjustable parameter tables can be dispersivelyprovided at a plurality of key points in an image processing flowchartto create each table according to a relatively simple target or rule, sothat image processing parameters can easily be obtained to reproducegradation and color well.

Because image processing controlling means are provided which switchinput γ characteristics set for ordinary copying to linear input γcharacteristics during preprocessing preceding patch data extraction,the present invention makes it possible to estimate the area ratenecessary for table creation with high accuracy when a patch image isread using a scanner to create pulse width selection table and agradation correction table.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A color image processing apparatus comprising: patch image outputmeans for outputting patch image data generated from patch data storedin storage areas; patch data extracting means for receiving read dataobtained by reading, by means of a scanner, a patch image which isformed corresponding to the patch image data output from the patch imagedata output means, and extracting a read patch data from the read data;gradation estimating means for estimating a record gradation of thepatch data on the basis of the read patch data extracted by the patchdata extracting means; and correction output means for correcting aninput color image signal on the basis of the record gradation estimatedby the gradation estimating means and the patch data stored in thestorage area, and for outputting the corrected signal.
 2. A color imageprocessing apparatus according to claim 1, wherein the gradationestimating means and correction output means include: pulse widthselection table setting means for estimating the record gradation of thepatch data on the basis of the read patch data extracted by the patchdata extracting means and for setting a pulse width selection table onthe basis of the estimated record gradation and the patch data stored inthe storage area; and pulse width output means for selecting a pulsewidth based on the pulse width selection table set by the pulse widthselection table setting means, and for outputting an input color imagesignal in accordance with the selected pulse width.
 3. A color imageprocessing apparatus according to claim 2, wherein the pulse widthselection table setting means includes pulse width selection tablesetting means for pre-processing the read patch data extracted by thepatch data extracting means after changing ordinary input γcharacteristic settings to linear settings, for estimating the recordgradation of the patch data from the preprocessed read patch data, andfor setting a pulse width selection table on the basis of the estimatedrecord gradation and the patch data stored in the storage area.
 4. Acolor image processing apparatus according to claim 1, wherein thegradation estimating means and correction output means include:gradation correction table setting means for estimating the recordgradation of the read patch data on the basis of the read patch dataextracted by the patch data extracting means, and for setting agradation correction table on the basis of the estimated recordgradation and the patch data stored in the storage area; and gradationcorrection output means for correcting the gradation of an input colorimage signal on the basis of the gradation correction table set by thegradation correction table setting means, and for outputting thecorrected color image signal.
 5. A color image processing apparatusaccording to claim 4, wherein the gradation correction table settingmeans includes pulse width selection setting means for pre-processingthe read patch data extracted by the patch data extracting means afterchanging ordinary input γ characteristic settings to linear settings,for estimating the record gradation of the read patch data from thepreprocessed read patch data, and for setting a gradation correctiontable on the basis of the estimated record gradation and the patch datastored in the storage area.
 6. A color image processing apparatusaccording to claim 1, wherein the gradation estimating means andcorrection output means include: pulse width selection table settingmeans for estimating the record gradation of the read patch data on thebasis of the read patch data extracted by the patch data extractingmeans and for setting a pulse width selection table on the basis of theestimated record gradation and the patch data stored in the storagearea; second patch image output means for outputting second patch imagedata, by using the pulse width selection table set by the pulse widthselection table setting means, corresponding to the second patch datafor creating a gradation correction table which is stored in a secondstorage area; second patch data extracting means for receiving secondread data obtained by reading, by means of a scanner, a second patchimage which is formed corresponding to the second patch image dataoutput from the second patch image output means, and for extractingsecond read patch data from the second patch image; second gradationestimating means for estimating gradation of the second read patch dataon the basis of the second read patch data extracted by the second patchdata extracting means; gradation correction table setting means forsetting a gradation correction table on the basis of the recordgradation of the second read patch data estimated by the secondgradation estimating means and the second patch data; and correctionoutput means for correcting an input color image signal on the basis ofthe pulse width selection table set by the pulse width selection tablesetting means and the gradation correction table set by the gradationcorrection table setting means, and for outputting the corrected signal.7. A color image processing apparatus according to claim 1, wherein thegradation estimating means includes gradation estimating means forfinding C, M, Y, and K record area rates on the basis of the given readpatch data and for estimating the record gradation of the patch data onthe basis of the found C, M, Y and K record area rates.
 8. A color imageprocessing apparatus according to claim 1, wherein the gradationestimating means includes gradation estimating means for finding C, M,Y, and K record densities on the basis of the given read patch data andfor estimating the record gradation of the patch data on the basis ofthe found C, M, Y and K record densities.
 9. A color image processingapparatus according to claim 1, wherein the gradation estimating meansincludes gradation estimating means for estimating the record gradationof the patch data by receiving the estimated density from a densityestimator estimating the density and the area rate from an area rateestimator estimating the area rate, based on the given read patch data,and for outputting one of the estimated density and the area rate.
 10. Acolor image processing apparatus according to claim 1, wherein thegradation estimating means includes means for estimating recordgradation by receiving a plurality of pieces of estimated gradationinformation from a plurality of gradation estimators estimating recordgradation on the basis of the given read patch data, using referencetables, and by selecting and outputting one of the pieces of estimatedgradation information.
 11. A color image processing apparatuscomprising: first patch image output means for outputting first patchimage data on the basis of the first patch data for creating a pulsewidth selection table stored in the first storage area; first patch dataextracting means for reading, using a scanner, a first patch image whichis formed corresponding to first patch image data output from the firstpatch image output means, and for extracting first patch data inaccordance with the first patch image; first gradation estimating meansfor estimating record gradation of the first read patch data, from thefirst read patch data extracted by the first patch data extractingmeans; pulse width selection table setting means for setting a pulsewidth selection table on the basis of the record gradation of the firstread patch data estimated by the first gradation estimating means andthe first patch data; second patch image output means for outputtingsecond patch image data, using the pulse width selection table set bythe pulse width selection table setting means, corresponding to thesecond patch data for creating a gradation correction table stored inthe second storage area; second patch data extracting means for reading,using a scanner, a second patch image which is formed corresponding tothe second patch image data output from the second patch image outputmeans, and for extracting second read patch data in accordance with thesecond patch image; second gradation estimating means for estimating therecord gradation of the second read patch data on the basis of thesecond read patch data extracted by the second patch data extractingmeans; gradation correction table setting means for setting a gradationcorrection table on the basis of the record gradation of the second readpatch data estimated by the second gradation estimating means and thesecond patch data; third patch image output means for outputting thirdpatch image data, using the pulse width selection table set by the pulsewidth selection table setting means and the gradation correction tableset by the gradation correction table setting means and on the basis ofthe third patch data for creating a color conversion table stored in athird storage area; third patch data extracting means for reading, usinga scanner, a third patch image represented by third patch image dataoutput by the third patch image output means, and for extracting thirdpatch data according to the third read patch image; color conversiontable setting means for setting a color conversion table on the basis ofthe third read patch data extracted by the third patch data extractingmeans and the third patch data; and correction output means forcorrecting a given and input color image signal on the basis of thepulse width selection table set by the pulse width selection settingmeans, the gradation correction table set by the gradation correctiontable setting means, and the color conversion table set by the colorconversion table setting means, and for outputting the corrected signal.12. A color image processing apparatus according to claim 11, whereinthe pulse width selection table setting means include pulse widthselection setting means for pre-processing the read patch data extractedby the patch data extracting means after changing ordinary input γcharacteristic settings to linear settings, for estimating the recordgradation of the read patch data from the preprocessed read patch data,and for setting a gradation correction table on the basis of theestimated record gradation and the patch data stored in the storagearea; and wherein the gradation correction table setting means includegradation correction table setting means for pre-processing the readpatch data extracted by the patch data extracting means after changingordinary input γ characteristic settings to linear settings, forestimating the record gradation of the read patch data from thepreprocessed read patch data, and for setting a gradation correctiontable on the basis of the estimated record gradation and the patch datastored in the storage area.
 13. A color image processing apparatusaccording to claim 11, wherein the gradation estimating means includesgradation estimating means for finding C, M, Y, and K record area rateson the basis of the given read patch data and for estimating the recordgradation of the patch data from the area rates.
 14. A color imageprocessing apparatus according to claim 11, wherein the gradationestimating means includes gradation estimating means for finding C, M,Y, and K record densities on the basis of the given read patch data andfor estimating the record gradation of the patch data from thedensities.
 15. A color image processing apparatus according to claim 11,wherein at least one of the first and second gradation estimating meansincludes gradation estimating means for estimating the record gradationof the patch data by receiving the estimated density from a densityestimator estimating the density and on the basis of the given readpatch data and the area rate supplied from an area rate estimatorestimating the area rate, and for outputting one of the estimateddensity and the area rate.
 16. A color image processing apparatusaccording to claim 11, wherein at least one of the first and secondgradation estimating means includes means for estimating recordgradation by receiving a plurality of pieces of estimated gradationinformation from a plurality of gradation estimators estimating recordgradation from the given read patch data and selecting and outputtingone of the pieces, by using reference tables.
 17. A color image formingapparatus comprising: a color scanner for reading a color image from agiven original and for outputting color image data; a color printer forforming an image on a recording medium in accordance with the colorimage data; first patch image forming means for forming, using the colorprinter, first patch image on a recording medium in accordance with thefirst patch data for creating a pulse width selection table, which isstored in first storage area; first patch data extracting means forextracting first read patch data from the read image data obtained byreading, using the color scanner, the first patch image formed by thefirst patch image forming means; first gradation estimating means forestimating record gradation of the first read patch data on the basis ofthe first read patch data extracted by the first patch data extractingmeans; pulse width selection table setting means for setting a pulsewidth selection table on the basis of the record gradation of the firstpatch data estimated by the first gradation estimating means; secondpatch image forming means for forming a second patch image on arecording medium using the color printer and the pulse width selectiontable set by the pulse width selection table setting means on the basisof the second patch data for creating a gradation correction tablestored in the second storage area; second patch data extracting meansfor extracting second read patch data from read image data obtained byreading, using the color scanner, the second patch image output by thesecond patch image forming means; second gradation estimating means forestimating record gradation of the second read patch data on the basisof the second read patch data estimated by the second patch dataextracting means; gradation correction table setting means for setting agradation correction table on the basis of the record gradation of thesecond read patch data estimated by the second gradation estimatingmeans and the second patch data; third patch image output means forforming a third patch image on a recording medium with the colorprinter, using the pulse width selection table set by the pulse widthselection table setting means and the gradation correction table set bythe gradation correction table setting means on the basis of third patchdata for creating a color conversion table stored in the third storagearea; third patch data extracting means for extracting third read patchdata from read image data obtained by reading a third patch image formedby the third patch image forming means, using the color scanner; colorconversion table setting means for setting a color conversion table onthe basis of the third read patch data extracted by the third patch dataextracting means and the third patch data; correction output means forcorrecting an input color image signal on the basis of the pulse widthselection table set by the pulse width selection setting means, thegradation correction table set by the gradation correction table settingmeans, and the color conversion table set by the color conversion tablesetting means and output the corrected signal; and forming means forforming, using the color printer, an image on a recording medium inaccordance with the color image signal corrected and output by thecorrection output means.